826 
ZOOLOGY, 
example of this has been mentioned in the progression of 
fishes. These creatures contract the muscles of one side of 
their bodies, and thus bend the spine, which is highly elastic. 
These relax, and instantly its resilience strikes the tail against 
the water with astonishing quickness and force. The latter 
movement may be aided indeed by the muscles of the oppo¬ 
site side, but the chief power is exactly similar to that which 
a bow exerts when the hands of the archer having left the 
string, it brings this forward and propels the arrow. 
A great facility is afforded to the passage of animals along 
the ground by the media they inhabit. Thus the turtle, 
slow moving on land, can pass very quickly along the bot¬ 
tom of rivers; and the foot of some shell-fish is sufficient, 
when it is struck on the sea’s bottom, to afford the animals a 
considerable change of place. 
The usual division of the locomotive system of animals, 
has been into the class that have a skeleton; that is, a num¬ 
ber of solid levers, surrounded by muscles, and serving as 
points of resistance for the latter to act on, and into a class 
that have these points in the form of plates or scales that en¬ 
close the muscles. There are others, again, which have for 
points of resistance, soft parts that are too various to allow 
us to attempt to draw any general picture of their nature. 
They, none of them, possess, however, the rapid and auto¬ 
matic powers of motion that belong to such as have scales or 
bones. 
“ The simplest form of the external skeleton is seen in those 
shells which are termed univalves, and which answers no other 
purpose than to defend its inhabitant, which moves by the ac¬ 
tion of the soft parts. The bivalves contract themselves on the 
fluid, which they admit when open, and thus propel themselves 
along, or turn themselves over, while others protrude their soft 
parts, and use these only for progression. The Crustacea, and 
insects which have more complex movement, are covered by 
shells or scales united by joints, which enable these parts to 
move on each other, either in the manner of flexion, extension, 
rotation, or circumduction. The external skeleton of some 
animals, the earth-worm for instance, is composed of liga¬ 
mentous rings, that serve at once to defend the soft parts, and 
afford attachment to the locomotive powers. Horny cuticle 
serves the same purposes in many land insects. The internal 
skeleton may be composed of cartilages, which may be called 
soft bones, being very like them in structure, or of bones 
that vary much in strength and density, in different parts and 
in different animals. The most simple bony skeleton that is 
met with, is where the brain and spinal marrow only are 
enclosed in a bony case, composed of a number of bones 
closely connected with one another, hut readily admitting 
of motion. The skeletons of some fishes, the common eel 
for example, are of this kind. The skull incloses the brain ; 
it also forms a defence for the organs of sense, in which the 
tongue may be included; and the vertebrae that compose the 
spine, form a canal of bone in which the spinal marrow is 
contained. These are the principal, although not absolutely 
all the bones of these fishes; and it is by their means the 
progressive motion of the animal is performed. The most 
simple addition to this skeleton is the processes of the ver¬ 
tebrae being elongated in different degrees in the various 
kinds of flat fishes; and having their bones connected to 
the extremities belonging to the fins, as in the turbot. The 
spine is also extended in many animals beyond what is ne¬ 
cessary to defend the spinal marrow, and then forms the tail, 
which answers a variety of purposes. As the first and essen¬ 
tial use is to defend the brain and its appendages, the second 
is to assist in respiration, and different bones are added for 
that purpose, rendering the skeleton still more complex. In 
fishes which breathe the air contained in water, there are 
strong cartilages, on which the gills are supported and kept 
apart from each other. In animals that breathe the air of 
the atmosphere there are ribs; these, in the snake tribe, 
are only connected with the spine; the other extremities 
are loose, and are employed, in the progressive motion of 
the animal; but in birds and animals there is an addition 
to these, Ihe breast.bone. Although the ribs are in most 
animals employed entirely for the office of respiration, yet in 
the flying lizard the lower ribs on each side are extended fc 
a considerable length, and support the expanded membrane, 
which, in that animal, serves the purpose of wings. In the 
cobra-de-capello snake some of the ribs are so shaped as to 
support the hood from which the snake takes its name. The 
extremities form the next addition. In many sea-animals 
there are only two, called pectoral fins, as in the whale tribe, 
corresponding to the anterior extremities of quadrupeds. In 
others there are four, as in the seal and turtle, that come oc¬ 
casionally on shore, and in all land animals, as well as the 
inhabitants of the air, whose skeleton is formed of bone. 
The bones of the trank and four extremities form the essential 
part of the skeleton, for, in the most perfect animals, no new 
parts can be said to be added. In the different classes of 
animals they bear a much greater degree of resemblance than 
one would reasonably expect, from the uses to which they 
are applied, and the coverings in which they are enveloped; 
and there is no evident reason for this resemblance, unless it 
be intended to shew that all animals in this world come from 
the same hand, and belong to the same general scheme of 
creation. In what other way can we explain the fins of the 
whale, having nearly the same number of bones, and a si¬ 
milar form to those of the human hand, although the two 
parts in the living animals are so exceedingly unlike ? Or 
the still more remarkable circumstance which may be ob¬ 
served in the turtle, where there is even a resemblance of a 
thumb. Animals that walk, appear to have no general 
likeness in the form of their bodies to those that creep, and 
still less to those that fly; yet in all of them, however dif¬ 
ferently the bones are formed to adapt them to these various 
uses, the skeleton consists of a skull and four extremities which 
have a correspondence in their bones; so that, although the 
parts are fitted for uses not at all similar, they are all links of 
the same chain. The hones of the fin of the seal, which is 
formed for swimming, and those of the bat, which are made 
for flying, resemble the human hand, which has so different 
an office.” 
Bones send out processes, or are attached to hard and 
tough parts, which serve as weapons of attack and defence: 
as the sword of the sword-fish, the horns of cattle, the hoofs 
of horses, lions’ claws, &c. 
In order to render it possible to use bones as locomotive 
powers, it is necessary that they should be united by joints, 
so as to move readily on each other. “ Joints says Sir Eve- 
rard Home, “ are formed upon three different principles: the 
most simple is, where an elastic substance connects the two 
bones, and, by its thickness, and a mixture of oil in the in¬ 
terstices, is enabled to yield in any one direction, to the 
force applied to it, the oil being squeezed out of that part; 
the substance, like a sponge, recovering its former state as 
soon as the force is removed. The second is, where the two 
bones are kept apart from each other, by means of a liquid, 
or soft jelly, which serves as a centre, round which the 
motion takes place. The third, where the bones are covered 
with articulating cartilages, adapted to one another in form, 
and lubricated with a fluid, which makes them move with 
more facility upon each other. The first of these structures, 
which is the most simple that can be imagined, is rarely em¬ 
ployed ; it is, however, met with in the whalebone whale. 
Between the condyles of the lower jaw, and the basis of the 
skull, is interposed a thick substance, made up of a network 
of ligamentous fibres, the interstices of which are filled 
with oil, so that the parts move readily on each other. The 
condyles have neither a smooth surface, nor a cartilaginous 
covering; but are firmly attached to the intermediate sub¬ 
stance, which, in this animal, is a substitute for the double 
joint met with in the quadruped, and is certainly a substitute 
of the most simple kind. The second of these structures, where 
a fluid is interposed between the ends of the bones, which are 
hollowed out to receive it, forming a ball-and-socket joint, is 
met with between the vertebrae of fishes in general, and some¬ 
thing like this occurs in the vertebrae of man and the mammalia.” 
In these, however, the vertebral column is little hollowed. 
Hence caps of cartilages unite the edges of the bones, and these 
caps gradually decreasing in tenacity, become, in their centre. 
